The illegal use of cannabis continues to be widespread throughout the world, with over 7 million weekly users in the U.S. alone. While cannabis is generally perceived by users as a relatively benign substance, abuse leading to dependence is on a par with other drugs of addiction (e.g. cocaine, alcohol) and large cross-sectional studies of cannabis users have also indicated that up to 30% meet the DSM-IV criteria for dependence. Despite an increasing understanding of the pharmacological basis of cannabis' effects, relatively little research has examined the cognitive effects of cannabis consumption, in particular the influence the drug has on executive function. These effects are of interest due to the research suggesting that a dysexecutive loss of control contributes to the inability to control one's own behavior in the context of the strong motivation to consume a drug. The neurophysiological changes related to chronic cannabis use also appear to reflect the neurobiological processes underlying other types of drug addiction. Alterations to cerebral metabolism and structure resulting from chronic cannabis use are located in the neural mechanisms of cognitive processes that are argued to contribute to the progression and maintenance of drug abuse. Critically, it is contended here that cognitive executive processes are compromised in chronic cannabis users, which may facilitate the continuation of the drug consumption, and are also compromised acutely following cannabis withdrawal, making them susceptible to relapse during this period. To begin to address these issues, two studies are proposed that will examine aspects of the executive control of behavior considered germane to drug addiction. As well as addressing the basic cognitive phenomena, these studies will examine the neuroanatomical substrates of these (dys)functions, using functional MRI. Aims of the research include: to determine if dependent cannabis users are impaired in executive control of behavior relative to controls, and if users show specific impairments on these same cognitive operations following an acute withdrawal from cannabis relative to their own baseline performance. Aspects of executive control will include the ability to (a) exercise control over strong prepotent urges; and (b) maintain behavioural control in the face of increasing executive demands. A further aim is to examine whether a change in corticolimibic regions activation can be observed as a result of prolonged cannabis use, and whether a brain-behaviour relationship can be established between activation in these regions following acute withdrawal from the drum and performance on tests of executive control.